The present invention relates to improvements to a flat-flame burner intended for equipping reheat, holding or heat-treatment furnaces, in particular for iron and steel products, so as to lower its NOx production appreciably.
In order to properly understand the technical field to which the improvements forming the subject of the present invention apply, as well as the corresponding prior art, reference will firstly be made to FIGS. 1 to 4 of the appended drawings.
FIG. 1 therefore illustrates an embodiment of a prior art furnace for reheating iron and steel products, with top and bottom heating. The products to be reheated, denoted by the reference 1, are supported and transported within the furnace by a system of fixed and walking beams 2 and 3. The walking beams are moved in a motion comprising a rectangular cycle by virtue of the conjugate actions of a translation frame 4 and a lifting frame 5, in an arrangement well known to those skilled in the art. The furnace is produced in the form of a thermally insulated chamber 6 in which long-flame burners 7 and flat-flame burners 8 are placed, the latter burners being fitted into the roof of the furnace. The present invention relates to improvements to the flat-flame burners 8.
FIGS. 2 and 3 illustrate two embodiments of roof burners according to the prior art.
Shown schematically in the FIG. 2 at 9 is the combustion tunnel of a burner which has a flared opening, the shape of which is substantially in the form of a quadrant of a circle so as to propagate the stream of air and the flame of the burner along the profile of the combustion tunnel, by the Coanda effect, and along the plane P of the roof. The burner is fed with combustion air, which may or may not be preheated, via a feed pipe 10 and this air is distributed in the body of the burner through orifices 11 made in the air distributor, these orifices causing the combustion air to swirl so that this air flows helically around the fuel-injection pipe 12. The latter lies along the axis of the burner so as to bring the fuel or fuels into a zone conducive to obtaining good mixing with the combustion air. Introduction of the fuel or fuels takes place through one or more orifices 14 so as to obtain the flow portrayed by the arrow 15 in this FIG. 2.
A disc 13 is provided on the injection end of the pipe 12, the function of this disc 13 being to force the combustion air to be pressed against the internal wall of the combustion tunnel 9 so as to promote the formation of a flat flame and create a suction vortex for the combustion gases in the burner head. In FIG. 2, this vortex is portrayed by the arrow 16. The combustion gases within the chamber of the furnace are therefore recirculated at the burner head by induction of the vortex 16 created by the high-speed circulation of the air/gas mixture coming from the burner. The flame produced by this air-gas mixture spreads, as at 17, following the profile of the combustion tunnel 9 and the plane P of the roof of the furnace.
According to the prior art (FIG. 3), the roof burners may also be provided with twin fuel-injection pipes 18 and 19 having respective injection orifices 20 and 14. Moreover, this known type of burner is similar to the burner forming the subject of FIG. 2, the twin injection pipe allowing the use of two different types of fuel. A single injection of fuel via the orifices 20 may be employed, for example during the burner ignition phase, allowing better attachment of the flame at low fuel rates, particularly when the temperature of the furnace chamber is less than 750xc2x0 C. (no spontaneous ignition of the mixture).
Until recently, the prior art of the flat-flame burner illustrated in FIGS. 2 and 3 was technically satisfactory from the standpoint of controlling the flame geometry and the heat flux distribution. The technique according to the prior art was optimized entirely according to combustion criteria for the purpose of obtaining an intensive flame of suitable shape. In this approach, the emission of pollutants, particularly of NOx, was regarded as secondary.
The trend in local, European and world-wide regulations has forced operators to reduce NOx emissions from their plants. Research on burner design has incorporated this constraint, particularly in the case of flat-flame burners which generate much greater amounts of NOx than long-flame burners and which have formed the subject of extensive research and numerous improvements for the purpose of limiting their discharge.
It is known that the production of NOx gases in a flame depends on its temperature and on the oxygen partial pressure in the reaction zone of this flame. In particular, it is known that the amount of NOx produced increases significantly for flame temperatures greater than 1200xc2x0 C. All research on reduction of NOx products has therefore been carried out so as to reduce the temperature of the burner flame and to increase the volume of its reaction zone, particularly by diluting it with the combustion products contained within the furnace chamber and recirculated at the burner head.
FIG. 4 of the appended drawings shows a burner according to the prior art, designed so as to reduce the amount of NOx produced. In this type of burner, the fuel is injected right at the very end of the combustion tunnel of the burner, into the vortex 16 of the combustion products. The burner has a fuel-injection pipe lying along its axis and emerging in the combustion tunnel via a number of radial injectors 14. By this means, the fuel is injected radially at high speed, through the said injectors 14, into the combustion air level with the tunnel in a zone in which the combustion air is diluted with the gases from the furnace environment. This high-speed fuel injection via a small number of radial injectors furthermore divides the flame into several xe2x80x9csmall flamesxe2x80x9d which are less intensive and whose total volume is increased with respect to a single flame.
Based on this prior art, the object of the present invention is to reduce the amount of NOx produced by flat-flame burners using the principle of flame dilution for the purpose of reducing its temperature and lowering the oxygen partial pressure in its reaction zone.
This technical problem is solved by a flat-flame burner having at least one fuel-injection pipe lying along the axis of the body of the burner and a combustion-air feed. The burner is characterized in that the fuel is introduced via the injection pipe or pipes, through one or more axial orifices lying in a plane close to the external plane of the combustion tunnel, into the combustion products so as to produce a first dilution of the fuel in these combustion products. The fuel/combustion products mixture thus obtained is diluted further in the combustion air.